Chemistry Reference
In-Depth Information
Noticeably, the majority of transition metal aqua complexes with sulfonate coun-
ter anions show that the sulfonate group cannot readily displace water from the
coordination sphere of the metal ion [
63
]. However, a stable solid can be yielded
when suitably soft metal cations are employed, including alkali ions, larger alka-
line earth ions, and silver(I). The common feature of these ions is that none of
them have stringent preferences with respect to coordination number or geometry.
As compared with the carbon atoms in the carboxylic groups, the central atoms
of the sulfonic and phosphonic acids are able to accommodate more than eight
electrons in the outer electron shell, which accounts for a greater bonding flex-
ibility in these groups [
64
-
66
]. Nonetheless, the proton of the sulfonic group is
more easily dissociable than the proton of the carboxylic and phosphonic func-
tional groups and thus in this respect the sulfonic acids are stronger than their
phosphonic and carboxylic counterparts. On the other hand, sulfonic acids can
fully deprotonate at very low p
K
a
, causing serious problem with their stability.
This implies the relatively weak coordination interactions between the sulfonate
anions and metal cations, which make the frameworks insufficiently robust to
sustain permanent porosity [
39
,
64
]. In comparison with their sulfonate and car-
boxylate analogs, metal phosphonates exhibit much higher thermal and chemi-
cal stability due to the strong affinity of organophosphonic linkers to metal ions,
making them promising in the fields of energy conversion, adsorption/separation,
catalysis, biotechnology, and so forth [
67
,
68
]. As to phosphonates and sulfonates,
the coordination chemistry is quite similar, though the corresponding coordina-
tion is less predictable owing to more possible ligating modes and three probable
states of protonation relative to the carboxylate bridging groups. Typical coordina-
tion modes between organic linkages and metal ions are illustrated in Fig.
2.6
. For
a single phosphonate/sulfonate group, each oxygen atom possesses the capacity
to bridge more than one metal center. While formally a single phosphonate/sul-
fonate oxygen atom can bind to three metal centers, more typically, the oxygen
R
R
R
R
P
P
P
P
O
O
O
O
O
O
O
O
O
O
O
O
M
M
M
M
M
M
R
R
C
C
O
O
O
O
M
M
Fig. 2.6
Typical coordination modes of phosphonates and carboxylates
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